1. Field of the Invention
This invention relates to a process for forming a deposited film containing germanium, above all a functional film, particularly a deposited film containing amorphous or crystalline germanium to be used for a semiconductor device, photosensitive device for electrophotography, line sensor for image input, image pickup device, etc.
2. Description of the Prior Art
For example, for formation of an amorphous silicon film, the vacuum deposition method, the plasma CVD method, the CVD method, the reactive sputtering method, the ion plating method, the optical and CVD method have been practiced. In general, the plasma CVD method has widely been used and industrialized.
However, for the deposited film constituted of amorphous silicon, there is room left for further improvement in its overall characteristics with respect to electrical properties, optical characteristics, and resistance to fatigue due to environmental conditions and repeated usage. Further, improvement in its production and mass production, including uniformity and reproducibility, is desired.
The reaction process in formation of an amorphous silicon deposited film according to the plasma CVD method is considerably more complicated than the CVD method of the prior art, and several ambiguities existed in the reaction mechanism. Also, there are involved a large number of parameters for formation of the deposited film (e.g. substrate temperature, flow rates and ratios of gases introduced, pressure during film formation, high frequency power, electrode structure, structure of the reaction vessel, gas discharging speed, plasma generation system, etc.). The plasma formed by combination of such a large number of parameters may sometimes become unstable and result in markedly bad affects on the deposited film formed. Besides, the parameters inherent in the film forming device must be chosen for each device, and it has been difficult under the present situation to uniformalize the production conditions. On the other hand, for achieving satisfactory electrical, optical or mechanical characteristics for an amorphous silicon film, it has been deemed best to form an amorphous silicon film according to the plasma CVD method under the present situation.
However, depending on the applied uses of the deposited film, since it is required to achieve a sufficiently large film area, uniformity in film thickness, unformity of film quality, and also to attempt to perform a mass production with reproducibility by a high speed film formation, enormous equipment capital becomes necessary for acquiring suitable mass production devices for forming amorphous silicon deposited films according to the plasma CVD method. Also management items for mass production thereof will become complicated to make the management tolerance narrower. These matters, and also the subtleties involved in adjustment of the devices, have been pointed out as the problems to be improved in the future. On the other hand, in conventional CVD method of the prior art, high temperature is required to be used and no deposited film having practical characteristics could be obtained.
As described above, in formation of amorphous silicon films, it has earnestly been desired to develop a formation process which can perform mass production by means of a low cost device while maintaining practical characteristics and uniformity. These considerations may also be applicable to other functional films such as silicon nitride film, silicon carbide film, silicon oxide film, etc.